A Parisian doing a spot of fishing in the Seine river was left gobsmacked after reeling in what looked like a piranha fish, it emerged on Tuesday. The fish was actually a pacu, known as 'ball-cutter', after its proclivity for biting off men's testicles.

This gives a whole new meaning to "the mouth of the Seine."

A stunned fisherman called the Paris river police after finding the toothy South American native on the end of his line, according to a statement by Paris police on Tuesday.

Skeptical, but interested, police asked him to send over a photo of the catch.

When he did, river cops were shocked to find it was none other than the pacu, a cousin of the piranha, which is nicknamed “ball-cutter”, after its taste for human testicles.

The testicle-biting pacu fished out of the Seine in Paris last Friday. Photo: Paris Prefecture de Police

A Paris police photo shows the pacu caught last week measured 31 cm, or just over 1 foot. Photo: Paris Prefecture de Police

"The pacu is not normally dangerous to people but it has quite a serious bite. There have been incidents in other countries, such as Papua New Guinea where some men have had their testicles bitten off,” he said.